Digital pen and paper systems—sometimes called pen computing systems—are known in which a document includes a position identification pattern made up of markings printed on the document which can be detected by a suitable detection system and used to distinguish different positions on the document.
It is known to use documents having such position identification markings in combination with a pen or other device having an imaging system, such as an infra red camera, within it, which is arranged to image a small area of the page close to the pen nib. The pen includes a processor having image processing capabilities and a memory and is triggered by a force sensor in the nib to record images from the camera as the pen is moved across the document. From these images the pen can determine the position of any marks or strokes made on the document by the pen. The markings can be stored either directly as graphic images, or perhaps as a sequence of positions on the document each with an associated time stamp, which can be passed from the pen to a suitable processor such as a personal computer.
The combination of the pen and the patterned paper allows, for example, forms with checkboxes on to be provided and the markings of the check boxes with the pen to be detected. In further applications the pen stokes recorded by the pen may be analysed to recognise handwriting characters. An example of a system which employs this type of digital paper is known from Anoto AB.
A problem can arise where the pen does not capture the pattern information correctly and therefore cannot tell where it is on the document. In this case, a warning may be issued to the user so they are aware that a stroke has not been captured correctly. In one known pen, sold under the name Logitech lo by Logitech, the pen includes a buzzer which issues a warning by making the pen vibrate whenever a predetermined number of consecutive image samples—typically 5—are lost. A sample is considered lost when the position on the document cannot be determined from the pattern in the image sample. The most usual cause of errors is a misprint of the pattern which reduces either the clarity, or the accuracy of position, of the markings, although errors can occur if the pen is moved too quickly for samples to be captured. Repeated warnings are undesirable as they reduce the confidence of the user in the device. It is therefore desirable to reduce the number of errors that are reported. This has been achieved in the past in several ways.
One way of reducing the number of warnings issued is to allow the pen to lose a number of samples within a stroke without issuing a warning. Typically a warning is only issued if five consecutive samples are lost.
Other solutions have been based on increasing the quality of the pattern printing or searching for more robust pattern types. Improving the visibility of the pattern can be achieved by using inks with different frequency absorption to other markings on the document so that nothing obscures the pattern and the pen can always detect it. However this complicates printing of the documents. Using more accurate printing techniques to print the pattern markings also helps but increases the cost of printing.